A problem of fundamental importance is the nature of the transduction of physical changes in the environment to electrical activity in a receptor neuron. In the case of photoreceptors, the question centers on the processes intervening between absorption of photons by the primary photopigment in the receptor cell membrane and permeability changes of the membrane. It is proposed that these processes be investigated by correlating changes in the absorbance of the primary photopigment and its associated photoproducts with electrical changes of the membrane of a model photoreceptor selected for its large size (the lateral eye of the barnacle B. eburneus). For this purpose, single cell spectrophotometry will be correlated with the electrical changes of the membrane examined with microelectrodes for recording, polarization, voltage clamp and measurement of internal ion activities. The effects of temperature changes, agents that interrupt the normal metabolism of cells and molecular probes injected into the photoreceptor cell interior will be examined. The goals of this study are: (1) to establish whether or not the latent period between quantum capture and conductance changes of the membrane are voltage dependent, (2) to examine the effect of the electric field across the membrane on the absorbance of the primary photopigment and its intermediates, (3) to examine the relation between the existence of photoproducts of the primary photopigment and conductance changes of the membrane, (4) to isolate the photopigment and its intermediates and to characterize them according to their absorption spectra in the visible and ultraviolet regions, (5) to obtain a non-electrical estimate of Ca ions entry into the cell interior or the release of sequestered internal Ca ions during illumination, and (6) to establish the relation between light adaptation and the amount of primary photopigment in the cell.